In particular during takeoff and landing, and also while flying at low altitude, birds may collide with an aircraft, such as an airplane, and may damage the aircraft
In the event of such a ‘bird strike’ the engines (such as the fan and/or compressor) may be damaged when the bird or generally other objects contacting the aircraft enter the intake of the engine. This can result in high costs for the repair or the replacement of the engine.
One approach is to design and to test the engine for a defined bird strike (i.e., an impinging object with maximum weight, maximum density, etc.). This may mean that the engine indeed may be damaged, but in spite of the bird strike does not fail during the flight. The engine can function with reduced output until a safe landing.